Electrical inverter



June 20, 1944. B. E. LENEHAN ELECTRICAL INVERTER Filed March 25, 1942 B W UR INVENTOR WITNESSES:

m w m mm r e 5 Patented June 20, 1944 2,351,982; v ELECTRICAL nwnn'raa Bernard E. Lenehan, to Westinghouse of Pennsylvania Bloomfield, n.1,, assignor Electric Company, East Pittsburgh,

4; Manufacturing Pa., a corporation Application March 25, 1942, Serial No. 436,077

11 Claims. (Cl. 177-351) This invention relates to systems for maintaining a predetermined relation between a pair of quantities, and it has particular relation to a system for producing an alternating-current quantity which bears a predetermined relation to a variable quantity.

In some cases it is desirable to produce an alternating-current quantity which bears a predetermined relation to some variable quantity. For example, such practice permits the adoption of sturdy and reliable alternating-current translating devices for response to variable quantities other than alternating-current quantities.

In accordance with the invention, an alternating-current quantity is produced which is balanced against a variable quantity to maintain a predetermined relation therebetween. A deviation from the predetermined relation between the quantities is employed for varying the charge in an energy storage device such as a capacitor. A charge on the capacitor is employed for controlling the balance of an alternating-current bridge. Unbalance of the bridge in either of two directions produces one of two alternating voltages differing 180 in phase. The output of the bridge circuit is employed for restoring the balance between the alternating-current quantity and the variable quantity.

It is, therefore, an object of the invention to provide a system for maintaining a predetermined relation between a variable quantity and an alternating-current quantity.

It is a further object of the invention to provide a system for maintaining a predetermined relation between a pair of quantities wherein the charge in an energy storage device is employed for controlling the predetermined relation between the quantities.

It is another object of the invention to provide a system for maintaining a predetermined relation between a pair of quantities wherein in response to a deviation of said quantities from said predetermined relation an alternating-current bridge may be unbalanced in either of two directions to provide compensating voltage outputs differing 180 in phase to restore the quantities to the predetermined relation.

Other objects of the invention will be apparent from the following description taken in conjunction with the accompanying drawing, in which the single figure is a schematic view of a system embodying the invention.

Referring to the drawing, there is shown a translating device I which is to be energized by an alternating-current quantity. In the specific embodiment herein discussed, the translating device may take the form of a conventional induction-type impulse transmitter. This transmitter may include a voltage winding 3 and a current winding 5 which are designed, when energized, to produce a shifting magnetic field. A conductive armature i is positioned for rotation in the shifting magnetic field on a shaft 9. A permanent magnet (not shown) is generally employed for damping rotation of the armature i.

To produce impulses proportional to the rate of rotation of the armature 1 a suitable commutator may be mounted on the shaft 9. This commutator may take the form of a cylinder ll of insulating material in which spaced conductive segments l3 are embedded. A pair of fixed brushes l5 are connected in series with a battery I! or other source of energy. When each of the conductive segments l3 bridges the brushes I5, an impulse is produced in a circuit l9 which may transmit the impulses to an impulse receiver 2|. The construction and operation of such an impulse transmitter are well understood in the art.

In order to energize the translating device i in accordance with a variable quantity, a balance device 23 is provided, having an alternating-current element 25 and a control element 21' which is energized in accordance with the variable quantity. These elements operate differentially on a balance arm 29 which is pivotally mounted on a stationary support 3|.

The alternating-current element 25 may be of the watt type having current windings 33 mounted on the balance arm 29 for movement relative to fixed voltage windings 35.

The alternating-current element 25 is energized in accordance with the energization of the translating device I. To this end, the current windings 33 and 5 of the alternating-current element and the translating device are connected in series for energization from a common source of alternating current. Although the voltage windings 35 and 3 of the alternatingcurrent element and the translating device may be energized from the same source as the current windings they are illustrated as connected in parallel for energization from a source 31 of alternating-current energy. This source may operate at any desired voltage and frequency. For the purpose of discussion, it is assumed that the source 31 is a 115-volt source operating at a frequency of cycles per second.

The control element 2'! may be energized in accordance with any variable quantity in order to control the energy supplied to the alternatingcurrent element 25 and the translating device I. For the purpose of discussion it is assumed that the control element is a direct-current element designed for energizatlon from a circuit 39. This circuit 39 represents a circuit 'from a currenttype telemetering transmitter. Current flowing in the circuit 39 may represent any desired quantity such as power or watts. In the conventional current type telemetering system, current flowing in the circuit 39 may be of the order of a few milliamperes in magnitude.

The control element 21 includes a permanent magnet II having pole pieces 99 and N. A winding 41 surrounds the pole l9 and is mounted for movement with respect thereto. As shown, the winding 41 is mounted on the balance arm 29 and is energized from the circuit 99.

The windings of the alternating-current element 25 and of the control element 21 are so connected that they apply opposing torques to the balance arm 29. If these torques are equal a movable contact 49 carried by the balance arm 29 may occupy a position intermediate two fixed contacts 5| and 53. If the current applied to the winding 41 decreases or rises in value, the resulting variation in torque applied to the balance arm 29 actuates the movable contact 49 into engagement with one of the contacts 5| and 53. The alternating-current element 25 and the control element 21 are of conventional construction and are well known in the art.

Movement of the movable contact 99 into engagement with either of the fixed contacts 5| or 53 is employed for modifying the alternating current supplied to the alternating-current element 25 and the translating device I to maintain a predetermined relation between the alternating current and current flowing in the circuit 39. For this purpose, movement of the movable contact is employed for controlling the balance of an alternating-current bridge circuit 55 having input terminals 51 and 59 connected across a secondary winding SI of a transformer 63. This transformer may have a primary winding 65 connected for energization from the source 31.

The bridge circuit 55 includes arms having resistances which vary as a function of the current flowing therethrough and having rectifiers associated therewith for permitting current to fiow through each arm in only one direction. Since barrier layer rectifiers, such as copperoxide rectifiers, have resistances which vary inversely with current density therein, such rectifiers may be employed in the arms of the bridge circuit, and four barrier layer rectifiers 61, 99, II and I3 are employed for this purpose. By inspection of the drawing, it will be observed that the rectifiers are connected to permit the flow of current in the same direction around the loop formed by the arms of the bridge circuit. The bridge circuit 55 has output terminals and I! connected across the primary winding I9 of a transformer 9i. If copper-oxide rectifiers are employed the maximum voltage per rectifier disc preferably should not exceed 0.3 volt. As a specific example, if the secondary winding 5| has an output of 6.3 volts, each of the rectifiers til, 99, II, and I3 may" comprise sixteen discs each in diameter to provide a voltage per disc of about 0.278 volt.

To provide the required unbalance of the bridge circuit 55, the system includes means for applying direct current to the terminals of the bridge circuit. Such direct current may be derived irom a conventional rectifier tube 83 having plates connected to the terminals of a secondary winding of the transformer 63 and having a filament 91 energized from a secondary winding 99 associated with the transformer 93. In accordance with conventional practice, a cen-- ter tap 9| is provided for the secondary winding 95 and is illustrated as connected to ground. The rectifier circuit illustrated provides fullwave rectification.

The output of the rectifier tube 99 passes through a filter of any desired type. In the specific filter illustrated, capacitors 93, inductive reactances 95, and a resistor 9'! are employed for removing ripples from the output of the rectifier tube 83.

For applying direct current to the bridge circuit 55, this bridge circuit is connected across the output terminals of a Wheatstone bridge circuit having one input terminal 99 connected to a terminal |0I of the filter and having a second input terminal represented by ground. The arms of this Wheatstone bridge circuit include resistors I03, I05 and I01. The fourth arm of the Wheatstone bridge circuit includes the plateto-cathode resistance of an electronic tube I09.

One output terminal III of the Wheatstone bridge circuit is connected to the output terminals oi the altemating-current bridge circuit 55 through a conductor H3, and a center tap I I5 of the primary winding 19.

The remaining output terminal of the Wheatrstone bridge circuit, represented by the conductors H0 which energize the filament of the tube I09, may be considered to be connected to both of the input terminals 51 and 59 of the alternating-current bridge 55.

From an inspection of the drawing, it will be observed that the direct-current voltage applied across the arms of the alternating-current bridge 55 is controlled by the bias of the control grid ill of the electronic tube I09. If the current flowing through the electronic tube I09 is such that the Wheatstone bridge formed by the tube and resistors I3, I05 and I0! is balanced, no direct-current voltage appears across the rectifiers 51, 69, II and 13 of the alternating-current bridge circuit.

If the bias on the tube I09 is such that the Wheatstone bridge is unbalanced in a direction making the output terminal III of the bridge positive with respect to the remaining output terminal of the Wheatstone bridge circuit, direct current fiows through the rectifiers 59 and 13.

Since the barrier layer rectifiers 69 and 13 have resistances which vary inversely with the density of current therein, the fiow of direct current therethrough lowers the resistance of the barrier layer rectifiers 59 and I3 below that of the remaining rectifiers. Consequently, the altemating-current bridge circuit 55 is unbalanced and an alternating-current voltage appears across the primary winding 19.

Let it be assumed next that the Wheatstone bridge circuit represented by the tube I09 and the resistors I03, I05 and i0! is unbalanced in a direction such that the output terminal ill is negative with respect to the remaining output terminal of the Wheatstone bridge circuit. In such a case, direct current fiows through the rectifiers 91 and II. As the result of this flow of direct current, the resistance of the rectifiers 61 aaugoaa and 1| falls to a value lower than that of the rectiners 33 and 13. Consequently, the alternating-current bridge circuit 59 is unbalanced in the opposite direction and an alternating-current voltage appears across the primary winding 19. Depending on the direction of unbalance of the alternating-current bridge circuit 55, one 01' two alternating-current voltages which differ in 1:111:24; by 180 appears across the primary wind- The electronic tube I09 may be of any suitable amplifying type. As illustrated. the tube I09 has a filament I'I9 energized from the secondary winding 9| over the conductors IIO through a suitable resistor I20. A cathode I 2! is connected to a center tap I23 on a resistor I25 which is connected across the terminals of the filament I I9. The. plate I21 and screen grid I29 of the tube I99 are connected to the terminal IOI' of the filter.

The bias on the control grid I I1 01' the electronic tube I09 is controlled by the charge on an energy storage device such as a capacitor I3I. This capacitor has a charge which depends on the position of the movable contact 49 of the balance device 23. When the movable contact 49 engages the fixed contact 5I, the capacitor I 3| is connected across the output of the filter through a resistor I33. Under these conditions, the resistor I33 permits the charge on the capacitor Hi to increase slowly. The increase in charge of the capacitor I'3I is accompanied-by an increase in the positive bias of the tube I09.

When the movable contact 49 engages the fixed contact 53, the capacitor I 3I is connected across a resistor I35. This resistor permits the charge in the capacitor I3I to decrease slowly, thereby decreasing the positive bias of the electron tube I09. Preferably. the capacitor I3I is of a high quality type capable of retaining its charge for a long period of time.

From this discussion. it is clear that the balance of the Wheatstone bridge circuit formed by the tube I09 and the resistors I03, I05 and I01 is controlled by the movable contact 49 of the current balance device. The constants of the system may be so selected that current flows in the plate circuit of the tube I09 over the entire range of operation of the system. When the bias on the tube I09 drops below a predetermined value, the alternating-current bridge circuit 55 is unbalanced in one direction to apply a first alterhating-current voltage across the primary winding 19. When the bias applied to the tube I09 rises above the predetermined value, the alternating-current bridge circuit 55 is unbalanced in an opposite direction to apply a second alternating-current ,voltage across the primary winding 19 which differs in phase by 180 from the first alternating-current voltage. The magnitude of the altemating-current voltage in each case depends on the magnitude of the deviation of the bias on the tube I09 from the predetermined value thereof.

The output of the transformer 8| may be employed for modifying the energization of the translating device I and the alternating-current element 25. For this purpose, the secondary winding I31 of the transformer 8| may have its terminals connected respectively to the control grids I39 and HI of a pair of electronic tubes I43 arranged for push-pull amplification. As illustrated, a capacitor I45 and a pair of resistors I41 are connected across the secondary winding I31. The capacitor I'45 may be employed to -ings 33 and 5 in the output of the transformer II and to provide a proper phase relation for the output of the tubes I43. It may be proportioned to be approximately in resonance with the inductance of the winding I 31. The resistors I41 each have a terminal connected to ground and to the cathodes I49 of the electronic tubes I43 through a resistor I5I. The filaments of the electronic tubes I43 may be energized through a suitable resistor I53 from a secondary winding I55 associated with the transformer 53. The tubes I43 include screen grids I 51 which are connected to the terminal IOI oi the filter.

The plate circuit for the tubes I43 is completed by the primary winding I59 of an output transformer I6I having a center tap connected through a conductor I 93 and the inductive reactor 95 to the filament 81 of the rectifier tube. A capacitor I 65 is connected across the primary winding I59. The capacitor I65 may be proportimed to correct the power factor of the load circuit of the tubes I43 to This assures-a desirable load for the tubes.

Preferably, the amplifier tubes I 43 have linear amplifying characteristics and produce in the secondary winding I61 of the output transformer an alternating-current voltage which is an amplified replica of the voltage applied to the grids of the tubes.

Although the output of the secondary winding I61 may be applied directly to the current windof the alternating-current element 25 andthe translating device I, preferably a portion of the energization of these windings is derived directly from a suitable source such as the secondary winding I 55 through a suitable impedance, such as resistor I66. In a specific embodiment illustrated in the drawing, the output of the secondary winding I61 serves to buck or boost the output of the secondary winding I55 dependent on the direction of unbalance of the al ernating-current bridge circuit 55.

It is believed that the operation of the system illustrated in the drawing is apparent from the foregoing discussion. As previously explained the control element 21 is energized in accordance with the variable control quantity which in this case is direct current. The alternating-current element 25 and the impulse generator I both are energized in accordance with the same alternating-current quantity. As long as the alternating-current element 25 and the direct-current control element 21 are balanced, the movable contact 49 remains in its central position and the impulse enerator I continues to send out impulses at a uniform rate. If the direct-current quantity supplied through the circuit 39 should increase. the balance of the balance device 23 is destroyed and the movable contact 49 moves into engagement with the fixed contact 5|. Such movement of the movable contact connects the capacitor I 3i across the filter through the resistor I33. Consequently, the charge in the capacitor I3I increases.

Should the current flowing in the circuit 39 decrease, the balance of the balance device 23 also is destroyed and the movable contact 49 moves into engagement with the fixed contact 53. Engagement of the fixed contact 53 by the movable contact 49 connects the capacitor I3I across the resistor I35 and permits the capacitor todischarge.

A change in the charge in the capacitor I3I varies the bias of the electronic tube I09 and opreduce harmonics winding I 55.

erates to vary the balance of thewheatstone bridge formed by the tube I! and resistors I 03, I08 and I0! in either of two directions dependent on the direction of variation in charge 01 the capacitor I ll from a predetermined value.

Since the alternating-current bridge 55 is connected across the output terminals 01 the Wheatstone bridge formed by the tube I09 oi' the resistors I 03, I and I01, a direct-current voltage is applied across the alternating-current bridge having a polarity dependent on the direction of unbalance oi the Wheatstone bridge. Depending on the polarity of the direct-current voltage thereacross, the alternating-current bridge circuit 55 has one of two alternatingcurrent outputs varying in phase by 180".

The alternating voltage output of the alternating bridge circuit 55 is applied-to the two tubes I43 which are connected in a push-pull amplifying circuit. The output of the tubes I 43 is coupled through the transformer IN to the secondary winding I55 which energizes the windings of the alternating-current element 25 and the winding 5 of the impulse generator I. De pending on its phase which is determined by the direct-current voltage across the alternatingcurrent bridge 55, the output of the tubes. I43 bucks or boosts the output of the secondary This varies the energization of the windings 33 in a direction tending to restore the balance of the balance device 23. When balance is restored, the movable contact 48 returns to its central position and the altemating-current element 25 continues to be energized at the new level. Since the impulse generator I is ener gized in accordance with the energization of the alternating-current element 25, the impulses generated thereby are in accordance with the new level of energization of the altemating-current element. In this manner, the output of the impulse generator I bears a predetermined relation to the direct current supplied from the circuitll.

It should be observed that the output of the tubes I43 operates to buck or boost the output of the secondary winding I 56 dependent on the polarity of the direct-current voltage across the alternating-current bridge 55. This permits the full output of the tubes to be utilized in either the bucking or boosting direction and results in eilicient utilization of the tubes. It should be observed additionally that the system illustrated in the drawing responds promptly to a change in current flowing through the circuit 30. For this reason, the output or the impulse generator i accurately represents the current flowing through the circuit 39.

As well understood in the art, "the various parts of the system illustrated in the drawing may vary appreciably. As specific examples, the tube I09 may be a type 6L7 tube which is known in the art as a penta-grid mixer amplifier tube. Each of the resistors I03, I05 and I0! which cooperate with the tube I 00 to form a Wheatstone bridge circuit, may have a resistance value of 20.000 ohms. The capacitor I3l may have a capacitance of tour microfarads. As previously pointed out, it is desirable that the capacitor Ill be of high quality. The resistors I33 and I35 through which the capacitor I3I is charged and discharged may each have a resistance value of 5 megohms. Type 6L6 tubes may be employed as the amplifying tubes I. Since the remaining constants of the system are dictated largely by the characteristics of the tubes selected, a

further discussion thereof is believed to be unnecessary. I! desired, the system may be housed in two electroconductive shielding cabinets A and B, which are both grounded.

Although the invention has been discussed with reference to a specific embodiment thereof,

numerous modifications are possible. Therefore, the invention is to be restricted only by the appended claims as interpreted in view of the prior art.

I claim as my invention:

1. In an electrical system for maintaining a predetermined relation between a first variable quantity and a second quantity, a bridge circuit having input terminals and output terminals, said bridge circuit having arms including elements having current-directional characteristics and having impedances which var with current, a source of alternating-current connected to said input terminals, means for passing direct current through said arms for unbalancing said bridge in a direction and to a magnitude dependent respectively o" the direction and magnitude of a deviation of aid quantities from a predetermined relation, and means responsive to the output of said bridge circuit for restoring said quantities to a predetermined relation.

2. In an electrical system for maintaining a predetermined relation between a first variable quantity and a second quantity, a plurality of barrier-layer rectiflers, means connecting said rectifiers in a bridge circuit having input and output terminals, said rectifiers forming two unidirectional, oppositely-directed paths between said output terminals, a source of alternating current connected to said output terminals, said bridge circuit being designed when unbalanced to provide an alternating-current output having a phase relationship relative to the voltage or said source which varies in accordance with the specific direction of unbalance of the bridge circuit, means for applying between the input terminals and the output terminals of the bridge circuit a direct-current voltage varying in accordance with said first variable quantity, whereby said directcurrent voltages produce a current flow through two arms of said bridge circuit which varies the resistance of said arms and unbalances said bridge circuit, and means responsive to the output of said bridge circuit for restoring said quantities to said predetermined relation.

3. In an electrical system for maintaining a predetermined relation between a first variable quantity and a second quantity, a plurality of barrier-layer rectiflers, means connecting said rectiflers in a bridge circuit having input and output terminals, said rectiflers forming two unidirectional, oppositely-directed paths between said output terminals, a source of alternating current connected to said output terminals, said bridge circuit being designed when unbalanced to provide an alternating-current output having a phase relationship relative to the voltage of said source which varies in accordance with the specific direction of unbalance of the bridge circuit, means for applying between the input terminals and the output terminals of the bridge circuit a direct-current voltage having a polarity and a magnitude which vary respectively in accordance with the direction and magnitude of the deviation of said quantities from a predetermined relation thereof, whereby said direct-current voltage produces a current flow through certain arms of said bridge circuit to vary the impedance of said certain arms and unbalance the bridge circuit in assnaaa a direction dependent on the polarity of said direct-current voltage, said bridge circuit having alternating-current outputs diiiering in phase by 180 electrical degrees in dependence on the direction unbalance of said bridge circuit, and means responsive to the alternating-current output oi. said bridge circuit for restoring said quantitles to said predetermined relation.

4. In an electrical system for maintaining a predetermined relation between a first variable quantity and a second quantity, a plurality of barrier-layer rectiilers, means connecting said rectifiers in a bridge circuit having input and output terminals, said rectifiers forming two unidirectional, oppositely-directed paths between said output terminals, a source of alternating current connected to said output terminals, said bridge circuit being designed when unbalanced to provide an alternating-current output having a phase relationship relative to the voltage of said source which varies in accordance with the specific direction of unbalance of the bridge circuit, means for applying between the input terminals and the output terminals of the bridge circuit a direct-current voltage having a polarity and a magnitude which vary respectively in accordance with the direction and magmtude oi the deviation of said quantities from a predetermined relation,

.whereby said direct-current voltage produces a current fiow through certain arms of said bridge circuit to vary the impedance of said certain arms and unbalance the bridge circuit in a direction dependent on the polarity of said direct-current voltage, said last-named means comprising a capacitor, means for varying the charge in said capacitor in accordance with the deviation of said quantities from said predetermined relation, and means responsive to the charge in said capacitor for producing said direct-current voltage, said bridge circuit having alternating-current outputs dififering in phase by 180 electrical degrees in dependence on the direction of unbalance of said bridge circuit, and means responsive to the alternating-current output of said bridge circuit for restoring said quantities to said predetermined relation.

5. In an electrical system for maintaining a predetermined relation between a first variable quantity and a second quantity, a plurality of barrier-layer rectifiers, means connecting said rectifiers in a bridge circuit having input and output terminals, said rectifiers forming two unidirectional, oppositely-directed paths between said output terminals, a source of alternating current connected to said output terminals, said bridge circuit being designed when unbalanced to provide an altemating-current output having a phase relationship relative to the voltage of said source which varies in accordance with the specific direction of unbalance oi the bridge circuit, means for applying between the input terminals and the output terminals of the bridge circuit a direct current voltage having a polarity and a magnitude which vary respectively in accordance with the direction and magnitude or the deviation of said quantities from a predeterminedrelation, whereby said direct-current voltage produces a current fiow through certain arms of said bridge circuit to vary the impedance of said certain arms and unbalance the bridge circuit in a direction dependent on the polarity 0! said direct-current voltage, said last-named means comprising a capacitor, means for varying the charge in said capacitor in accordance with the deviation in said quantities from said predetermined relation, an electronic device having .a direct-current output controlled by the charge in said capacitor, and means responsive to the direct-current output 01' said electronic device for producing said direct-current voltage, said bridge circuit having alternating-current outputs difi'ering in phase by electrical degrees in dependence on the direction of unbalance of said bridge circuit, and means responsive to the altemating-current output of said bridge circuit for restoring said quantities to a predetermined relation.

6. In an electrical system for maintaining a predetermined relation between a first variable quantity and a second quantity, a plurality of barrier-layer rectifiers, means connecting said rectifiers in a bridge circuit having input and output terminals, said rectifiers forming two unidirectional, oppositely-directed paths between said output terminals, a source oiv alternating current connected to said output terminals, said bridge circuit being designed when unbalanced to provide an alternating-current output having a phase relationship relative to the voltage of said source which varies in accordance with the specific direction of unbalance oi the bridge circuit, means for applying between the input terminals and the output terminals 01 the bridge circuit a direct-current voltage having a polarity and a magnitude which vary respectively in accordance with the direction and magnitude of the deviation of said quantities from a predetermined relation, whereby said direct-current voltage produces a current flow through certain arms 01' said bridge circuit to vary the impedance of said certain arms and unbalance the bridge circuit in a direction dependent on the polarity of said direct-current voltage, said last-named means comprising a capacitor, means for varying the charge in said capacitor in accordance with the deviation in said quantities from a predetermined relation, an electronic device having a direct-current output controlled by a grid element, means connecting said grid element for biasing in accordance with the charge in said capacitor, means producing a first direct voltage controlled by the direct-current output of said electronic device, means for producing a. second substantially constant second direct voltage, and connecting means for applying the difference between said first and second direct voltages between the output and input terminals of said bridge circuit.

7. In an electrical system for producing an altemating-current quantity controlled by a variable quantity, a capacitor, an electronic device having an output controlled by the bias of a grid element, means connecting said grid element for biasing in accordance with the charge in said capacitor, a bridge circuit having a pair of input terminals and a pair of output terminals, said bridge circuit comprising arms having barrier-layer rectifiers therein connected to permit the flow 01 current around said bridge circuit, means for applying an alternating voltage to said input terminals, means for applying between said pair of input terminals and said pair of output terminals 3. direct-current voltage dependent on the difference between said output of said electronic device and a substantially constant quantity, said direct-current voltage producing a direct-current fiow through said bridge circuit which cooperates with the resistance variation of said barrier-layer rectifiers with respect to current densities therein to unbalance said bridge circuit in either of two directions in dependence on the polarity oi said direct-current voltage, whereby-either of two outputalternating voltages diflering 180 in phase appears across the output terminals of said bridge circuit dependent on the polarity of said direct-current voltage, means for comparing said variable quantity to the alternating voltage appearing across the output terminals said bridge circuit, and means controlled by said comparing means in response to a deviation 0! said variable quantity and the alternating voltage output of said bridge circuit from a predetermined relation for varying the charge in said capacitor.

8. In an electrical system for producing an alternating-current quantity controlled by a variable quantity, a capacitor, an electronic device having an output controlled by the bias of a grid element, means connecting said grid element for biasing in accordance with the charge in said capacitor, a bridge circuit having a pair of input terminals and a pair of output terminals, said bridge circuit comprising arms having barrier-layer rectifiers therein connected to permit the flow of current around said bridge circuit, means for applying an alternating voltage to said input terminals, means for applying between said pair of input terminals and said pair of output terminals a direct-current voltage dependent on the difierence between said output of said electronic device and a substantially constant quantity, said direct-current voltage producing a direct-current flow through said bridge circuit which cooperates with the resistance variation of said barrier-layer rectifiers with respect to current densities therein to unbalance said bridge circuit in either of two directions in dependence on the polarity of said direct-current voltage, whereby either of two output alternating voltages differing 180 in phase appears across the output terminals of said bridge circuit dependent on the polarity of said direct-current voltage, said output alternating voltages having a magnitude dependent on the magnitude of said direct-current voltages, means combining the output voltage oi said bridge circuit with a substantially constant alternating voltage to produce a resultant voltage, means for comparing said variable quantity to said resultant voltage, and means controlled by said comparing means in response to a. deviation of said variable quantity and the said resultant voltage from a predetermined relation for varying the charge in said capacitor.

9. In an electrical system for producing an altemating-current quantity controlled by a variable quantity, a substantiall constant source oi. alternating current, a capacitor, an electronic device having an output controlled by the bias of a grid element, means connecting said grid element for biasing in accordance with the charge in said capacitor, a bridge circuit having a pair of input terminals and a pair of output terminals, said bridge circuit comprising arms having barrierlayer rectifiers therein connected to permit the flow of current around said bridge circuit, means for applying an alternating voltage to said input terminals, means for applying between said pair of input terminals and said pair of output terminals a direct-current voltage dependent on the difference between said output of said electronic device and a substantially constant quantity, said direct-current voltage producing a direct-current flow through said bridge circuit which cooperates with the resistance variation of said barrier-layer rectifiers with respect to current densities therein to unbalance said bridge circuit in either of two directions in dependence on the polarity of said direct-current voltage, whereby either oi two output alternating voltages diflering in phase appears across the output terminals oi said bridge circuit dependent on the polarity 01' said direct-current voltage, said output alternating voltages having a magnitude dependent on the magnitude of said direct-current voltage, means combining the output voltage of said bridge circuit with a substantially constant alternating voltage to produce a resultant voltage, a balance device comprising a control element responsive to said variable quantity and an alternating-current element opposing said control element, translating means, means for energizing said translating means and said alternating-current element in part from said substantially constant source of alternating current, means for energizing said translating means and said alternating-current element in part in accordance with said resultant voltage, and means controlled by said balance device for varying the charge in said capacitor.

10. In a device for producing alternating current controlled by a variable direct-current quantity, a capacitor, an electronic device having an output controlled by the bias of a grid element, means connecting said grid elementrfor biasing in accordance with the charge in said capacitor,- a bridge circuit having a pair of input terminals and a pair of output terminals, said bridge circuit comprising arms having barrier-layer rectifiers therein connected to permit the flow of current around said bridge circuit, means for applying an alternating voltage to said input terminals, means for applying between said pair of input terminals and said pair of output terminals a direct-current voltage dependent on the difference between said output of said electronic device and a substantially constant quantity, said direct-current voltage producing a direct-current flow through said bridge circuit which cooperates with the resistance variation of said barrier-layer rectifiers with respect to current densities therein to unbalance said bridge'circuit in either of two directions in dependence on the polarity of said direct-current voltage, whereby either of two output alternating voltages diii'ering 180 in phase appears across the output terminals of said bridge circuit dependent on the polarity of said direct-current voltage, said output alternating voltages having a magnitude dependent on the magnitude of said direct-current voltage, electronic amplifying means for amplifying the alternating voltage output of said bridge circuit, means combining the amplified output voltage of said bridge circuit with a, substantially constant alternating voltage to produce a resultant voltage having a magnitude dependent on the magnitude and phase of the alternating voltage output of said bridge circuit, a current balance device comprising a control element responsive to said variable direct-current quantity, an alternating-current element having a first energizing winding and a second energizing winding, and a balance element differentially responsive to said control element and said alternating-current element: an alternating current operated translating means having a first energizing winding and a second energizing winding; means for energizing said first windings from said substantially constant source of alternating current; means for energizing said second windings in accordance with said resultant voltage; and means controlled by said balance element ior connecting said capaeitortoaeourceoidirectcurrentortoadiecharge circuit d dent on the direction of deviation oi aaid tant voltage and said variable direct-current quantity from a predetermined retion. ii.

11. In a system for maintaining a predetermined relation between a variable quantity and an alternating-current quantity, a direct-current bridge circuit having input terminal; and output terminals, an alternating-current bridge circuit comprising current directional arm; having impedancee varying aa a runctlon or current flowing therethrough. means connecting said alternatin current bridge to raid direct-current bridge circult to be unbalanced in either of two directions depending on the polarity oi the voltage appearing across the output terminals or said directcurrent bridge circuit, means for applying an alternating voltage across the input terminals oi said alternating-current bridge circuit, and balance means responsive to a deviation of said variable quantity and the alternating output 01 said alternating-current bridge circuit from a predetermined relation for varying the balance oi said direct-current bridge circuit to maintain a predetermined relation between the alternating output of said alternating-current bridge circuit and said variable quantity.

BERNARD E. LENEHAN. 

